Error from Server.searchLdapUsers when SSO is authentication type

I have just setup a new vCO server and the only difference is this one is using SSO for authentication.  I have an action which uses Server.searchLdapUsers() and it is blowing up with this error

Wrapped java.lang.ClassCastException: ch.dunes.login.ldap.LdapGroup cannot be cast to ch.dunes.login.ldap.LdapUser (Dynamic Script Module name : QCfindLdapUserbyString#7)

Any clue as to why?

I am also using the beta build of the AD plugin but I don't think that is what is used for that call.

Paul